The in vivo 5-bromodeoxyuridine (BrdU)-dye method is particularly useful for cytogenetic analyses of tissue-specific levels of chromosome mutagenesis. Differences in baseline and mutagen-induced frequencies of sister chromatid exchange (SCE) for somatic and germ cells have been demonstrated. In the present study, in vivo approaches will be modified in order to extend SCE analyses to additional tissues. Further clarification of germ cell responsiveness and maturation following mutagen exposure will also be attempted.